Problem: $ C = \left[\begin{array}{r}1 \\ -1 \\ 2 \\ 1\end{array}\right]$ What are the dimensions of matrix $ C$ ?
Solution: The first dimension is the number of rows in the matrix. How many rows does $ C$ have? The second dimension is the number of columns in the matrix. How many columns does $ C$ have? So, $ C$ is a 4 $\times$ 1 matrix.